Field of the Invention
This invention applies to epitaxially grown solar cells and specifically to inverted solar cell structures. Inverted metamorphic III-V multi junction solar cells achieve the highest efficiencies (>30% in space and >40% terrestrial under concentrator). These cells are grown epitaxially on GaAs wafers that are up to 700 μm thick. The solar photons are absorbed in the epitaxial layer, which is about 10 μm thick. The substrate is only for mechanical support and considered wasted from a materials point of view. It is desired to lift-off the epi-layer and transfer to flexible polyimide substrate and to reuse the GaAs wafer to grow another epi-layer multiple times. The cost of materials to produce a GaAs wafer suitable for a solar cell is about $50 per Watt. This accounts for about 40% of the cost of the finished cell. There is a need to make high efficiency solar cells thin, lightweight and flexible to achieve high specific power (>200 W/Kg); and foldable so that they can be stowed in a small volume to increase payload space. A new cost-effective dry lift-off process which transfers the active cell layer wafer-scale to a less expensive flexible substrate without ion implantation or chemical etching is presented. This yields thin high efficiency solar cells which have the same performance on the host substrate as on the growth substrate and which are easily scalable to large size arrays. This process allows re-using of the base semiconductor wafer to grow new cells which results in savings in GaAs materials, grinding and etching expenses. This technology reduces the cost of the cell by 30% and allows a very rapid growth of the terrestrial market for high efficiency III-V solar cells.
The savings are not only monetary. This has huge implications on natural resources. Gallium is the rarest component of the photovoltaic compounds. All the newly developed solar cell materials, such as CIGS, contain Gallium. As the substrate must currently be ground away for the epitaxial lift-off, 12% of the world's current Ga production from mining goes to waste. This creates one of the most expensive toxic wastes because GaAs contains 48% Ga and 52% arsenic, which is toxic. Thus, cost is not the only issue but also availability of the material. Therefore, this technology solves the bottleneck in the Ga supply chain and will have broad impact on society and the ecology.
The base wafer accounts for 40% of the cost of a finished cell. Thus, reducing the materials used in solar cells will go a long way toward reducing the overall cost of the cell. This can be achieved by transferring the epi-layer to a cheaper substrate and reusing the wafer to grow another epi-layer. This would achieve the lowest cost, while preserving rare earth materials.
Discussion of Related Art
Wet epitaxial lift-off (WELO) has been demonstrated by incorporating a lattice-matched sacrificial release layer, such as Al0.7Ga0.3As, under the epi-layer, which gets etched off chemically in HCl or HF [1,2,3]. It takes up to several hours to free a GaAs layer from its growth substrate due to the restricted access of the etchant to the AlGaAs layer, especially for large size wafers. This is considered too slow for industrial utilization. It should take only seconds per wafer to be viable for large scale manufacturing. Therefore, a technique for quicker peel-off that would have wider industrial application is needed.
Solar cell manufacturers prefer to grow lattice-matched structures in order to minimize slips and dislocations which can degrade the performance of the solar cell. Nevertheless, thick epi-layers can be grown on strained layers, such as In0.3Ga0.7As, if the thickness of the strained layer is kept below the critical thickness, which for 2% strain is 50 mono-layers or about 10 nm. Lattice-matched InGaP etch stop layers are grown routinely under triple junction solar cells.
There were several recent attempts in the industry by IBM [4], IMEC [5,6] and AstroWatt [7] to slice a wafer from ingot kerflessly using a technique called “spalling”. These efforts concentrated mainly on silicon. A thick metallic stressor layer, usually Ni, is deposited on the semiconductor wafer and heated at very high temperature >800° C. The stress caused by the difference in the coefficients of thermal expansion between metal and semiconductor as the wafer cools to room temperature exceeds the toughness of the material and causes a crack to propagate in the substrate parallel to the surface at a certain depth below the interface. This causes a thin layer to separate from the base substrate and remain stuck to the metal film. A high peak temperature is necessary to cause the stresses. This enhances the diffusion of impurities from the metal into the silicon which act as recombination centers for electron-hole pairs and lower the efficiency [5,6]. Further, the high temperature affects the morphology of the crystal which causes the crack to propagate uncontrollably. As a result, the depth at which the cleavage occurs cannot be controlled precisely and the surface is rough. The metal layer is dissolved which yields a 25 μm free standing silicon foil. A solar cell is subsequently fabricated in the silicon foil. Foils up to 150 cm2 area have been fabricated [7]. The extracted foil is not supported and is therefore brittle and prone to chipping. Nevertheless, single junction solar cells fabricated in the foil exhibited similar performance to identical cells on bulk substrates which indicates that the quality of the crystal is not compromised by the lift-off. The highest efficiency that has been demonstrated on these foils is 15% [7].
The (110) is the preferred cleavage plane in GaAs. However, solar cell manufacturers are reluctant to grow on (110) wafers because most of the industry is built on (100) wafers, albeit with large off-cut angles up to 15° toward the (111)A plane.
The ultimate goal of all lift-off techniques is to re-use the base wafer after lift-off.
Laser micromachining inside the bulk of materials is not new. It is used for making waveguides inside transparent materials (glass) [10], and has been proposed as a way for wafering silicon from ingot [11]. These techniques depend on non-linear absorption at the focal point, forming a sub-surface defect while leaving the region between the surface and the defect untouched. Light from an IR laser at a wavelength above 1 μm is focused below the surface and scanned across the wafer. The size of the defect depends on the depth of focus. This requires the use of large diffraction limited lenses with short focal lens and low f# and necessitates the use of high precision and resolution vertical positioning stages, which are expensive. Silicon does not benefit from heterogeneous epitaxial growth where separation of the epi-layer from the wafer can be done using a sacrificial layer at the interface, and therefore, wafering in silicon relies more on the quality and precision of the optics rather than the physics of absorption.
The crystallographic structure of Si has been damaged by radiation in the NIR at 2256 nm with a threshold fluence of 0.18 J/cm2 using sub-picosecond pulses [12]. This radiation should also damage Ge because it has weaker covalent bonds having a lower bandgap. Laser ablation is also used for epitaxial lift-off of GaN layers from sapphire substrates for the fabrication of LEDs [13].
It is desired to cleave at or very near the epi/wafer interface without sacrificing substrate material. That would be true epitaxial lift-off so that the original wafer is recovered at full thickness. III-V materials have the advantage that devices are fabricated in epitaxially-grown layers. Thus, the epi-layer can be peeled off the substrate and transferred to a flexible carrier and the substrate can be reused to grow another epi-layer. The fragile crystalline solar cell must be supported at all times. A suitable flexible carrier is polyimide Kapton® sheet which is backed by DuPont and qualified for space applications. Kapton is available in sheets as thin as 50 μm which are easy to handle and come pre-coated with a uniform layer of acrylic adhesive. It operates continuously from cryogenic to >200° C. which is suitable for space applications. A Kapton sheet with adhesive layer is bonded to the epi-side of GaAs wafer using a hot roll laminator and then cured in an oven at 150° C.-190° C. for about an hour. After lift-off the thin epi-layer is carried by the Kapton sheet which serves as permanent carrier of the IMM solar cell. The entire thickness of the thin solar cell is less than 100 μm, which meets the specific power requirements for space.
FIG. 1 shows a thin IMM3J epi-layer carried by polyimide substrate. It shows a smooth shiny surface of the InGaP etch stop layer. This can be obtained by lifting-off the epi-layer without sacrificing the growth wafer so that it can be re-used to grow another epi-layer. This solar cell was rolled mechanically to a radius <½″ and shocked thermally without fracturing.
There are two ways to separate the epi-layer from the growth substrate kerflessly: either by 1) driving a crack at the interface due to thermal stresses, or 2) disintegrating a sacrificial layer by laser ablation. These two approaches involve different physics but lead to the same end result. Both are dry, meaning they are fast and there is no wet etching. These two concepts are illustrated in FIGS. 2a and 2b, respectively.
Dry Epitaxial Lift-Off (DELO)
The polyimide serves not only as the permanent carrier of the thin crystalline solar cell but also creates the thermal stresses that lead to cleavage. Kapton has a CTE of 18-20 ppm/° C. compared to 5-6 ppm/° C. for GaAs. The temperature is lowered by introducing the composite structure slowly in liquid nitrogen at −196° C. Within a minute, an audible crack initiates and the film snaps right off. A temperature differential ΔT˜300° C. (between the curing temperature and LN2) and a CTE difference of 12-14 ppm/° C. cause a stress of about 20 MPA which is sufficient to initiate a crack because it is amplified at the crack tip. Crack nucleation is due solely to the build-up of thermal stresses without applying external mechanical force. The crack propagates across the wafer in a fraction of a second. The wafer cleaves spontaneously at a plane parallel to the bond interface. For GaAs the lift-off temperature is about −140° C. whereas for Si the wafer must be cooled down to −196° C. The lift-off happens in a split-second and was captured on video. The front side of the solar cell can be processed on the Kapton after lift-off. The concept applies to all semiconductor materials, Si, Ge, GaAs and InP, and to all epitaxially grown solar cells for space as well as terrestrial applications. The concept is illustrated in FIG. 3.
The sequence of frames in FIG. 4 taken from http://www.youtube.com/watch?v=B1tj5ZUe9TI show the backside of a GaAs wafer piece bonded to Kapton 50 μm thick on the front side (underneath), on a plate as it is lowered slowly in a dewar of liquid nitrogen. The successive pictures show the drop in temperature over the course of a few minutes until lift-off happened at a temperature of −140° C. As the lift-off temperature was reached the Kapton layer snapped off almost instantaneously. The curled peeled off epi-layer stuck to the Kapton sheet (brown) is shown in FIG. 4d sitting on top of the GaAs wafer piece.
The first two video clips show the rolling and unrolling of thin GaAs epitaxial layers on Kapton as it is thermally cycled between a hot plate at +100° C. and −196° C. The third video shows the lift-off wafer scale. The last video shows the lifted-off layer held on a standard vacuum chuck spinning at 3000 RPM which allows fabrication of the front side of the solar cell. These movies take less than a minute each:    http://www.youtube.com/watch?v=WED8cj2YfIw 1:33 min rolling and unrolling    http://www.youtube.com/watch?v=U5rxiwkenmI 1:00 min rolling and unrolling    http://www.youtube.com/watch?v=BJr1LDdZabg 0:48 min wafer scale lift-off happens at 40 seconds    http://www.youtube.com/watch?v=VC6v7_RAOok 0:09 min GaAs on Kapton spinning at 3000 RPM
The polyimide is very advantageous because it is able to induce lift-off for a temperature drop ΔT of only 300° C. compared to metal which requires raising to >800° C. Furthermore, it is completely inert and does not contaminate the semiconductor layer. This process is low temperature between +200° C. and −196° C. The combination of polyimide/adhesive is tough and can withstand these temperatures. The acrylic adhesive holds in liquid nitrogen and is able to transmit the force and break-off a layer of semiconductor wafer without losing its grip. The adhesive layer is applied uniformly to Kapton. It has a smooth surface and supports the epi-layer over the entire 4″ wafer.
Cleavage by Crack Propagation Due to Thermal Stresses
The following is a summary of results obtained:
    1) lifted-off a layer of GaAs 10-15 μm thick equivalent to IMM3J layer thickness,    2) cleaved 2″ (110) and 4″ (100) orientation wafers and saved the base wafer as one piece,    3) reused a base wafer and lifted-off another layer several times, there did not appear to be a limit to the number of reuses as long as the substrate remained thick enough,    4) obtained atomically smooth (mirror-like) surfaces (single atomic plane cleavage, roughness <1 nm) over areas (>1 cm2) in (110) orientation wafer,    5) fabricated front side of solar cell on Kapton and demonstrated that the performance is not degraded compared to identical solar cell on GaAs wafer.FIGS. 5 and 6 show the lifted-off layer on Kapton to the right and the base wafer to the left for GaAs (110) and (100), respectively. Uniform cleavage is obtained across 4″ wafer. The (110) orientation yields smoother cleaved surface as expected. FIG. 7 shows AFM average roughness of 0.059 nm, below instrument noise, indicating that the cleavage is a single atomic plane.
A GaAs wafer bonded to polyimide substrate bends significantly in LN2 at −196° C. due to thermal stresses. Even at room temperature, there is significant residual stress and bow, as shown in FIG. 8. The thickness of the polyimide is optimized to deliver the maximum bending stresses. The highest stresses are obtained in the bending mode. The thin layer on Kapton rolls down to 0.75-inch diameter in LN2 without cracking, as shown in FIGS. 9 and 10. As the temperature drops, the polyimide shrinks at a higher rate than the GaAs. This causes the composite structure to bend and roll as a tube with the polyimide on the inside. The thin solar cell flattens when placed on a hot plate at 100° C.
The success of the epitaxial lift-off by crack propagation hinges on a delicate balance between the initial scratching and the loading by the polyimide which depends on the polyimide-to-GaAs thickness ratio and the crystallographic orientation of the wafer. GaAs (110) cleaves smoother and more uniformly than (100). The extent of the lateral crack propagation also depends on the thickness ratio. Thicker polyimide causes the crack to propagate farther but lifts-off a thicker layer and may break the wafer. It is desired to minimize the thickness of polyimide to increase the specific power ratio for space applications. The right combination of stresses is necessary to guide the crack near the interface. However, thermal stresses cause significant bow as illustrated in FIG. 2a. It is desired to minimize the effect of stresses on the performance of the solar cell.
Precise control over crack propagation is necessary to lift-off layers with uniform thickness and smooth surface. Atomically smooth cleaved surfaces were obtained by controlling the crack propagation. The polyimide applies pure bending moment on the GaAs wafer which is the optimal mode of opening a crack in tension (Mode I) because it avoids the shearing stresses (Mode III) which cause deviation in the path and uncontrollable crack propagation [8,9]. If these requirements are not met then the crack can bifurcate and branch out and cause secondary cracks to propagate at different angles along different paths. To maximize yield in production a fundamental understanding of fracture mechanics at the nano-scale is essential. For industrial applications this process must be controlled. The focus of the research is to better understand the stress mechanisms that lead to rupture and how to control the path of the crack. The challenges are controlling the crack propagation depth near the epi/wafer interface and preventing the substrate from shattering. Femtosecond laser ablation produces less severe stresses and a gentler lift-off than cooling in liquid nitrogen. Both concepts lead to recovery and reuse of the growth substrate.